CODEWITHSUNDEEP
riscv
Aneesh Raveendran
Aneesh Raveendran

Reputation: 391

RISC-V startup code for stack pointer initalization

how to write a simple startup code in C which initialize the stack pointer for RISC-V processor? (which have to compile thorugh risc-v compiler)

thanks

Upvotes: 0

Views: 7814

Answers (2)

Palmer Dabbelt
Palmer Dabbelt

Reputation: 1068

The simplest start code that sets up a stack looks something like

_start:
    /* Clear the link register, so debuggers know to terminate their
     * backtrace. */
    cfi_undefined (ra)

    /* Setup the global pointer, which the ABI assumes points to the
     * __global_pointer$ symbol. */
    .option push
    .option norelax
    la gp, __global_pointer$
    .option pop

    /* Setup the stack pointer.  This symbol will only be looked at
     * here, so you can pick any name you want. */
    la sp, __stack_pointer$

    /* Call main, leaving the arguments undefined. */
    call main

    /* Spin after main() returns. */
1:
    j 1b

you might also want to do something like calling __libc_init_array and registering __libc_fini_array with atexit, clearing argc and argv (a0 and a1 to main), relocating the data section, and zeroing the BSS, depending on exactly how much of the C ABI you want to support.

SiFive's Freedom E SDK contains our full startup code: https://raw.githubusercontent.com/sifive/freedom-e-sdk/master/bsp/env/start.S , as well as the relevant linker scripts for some of our platforms. You can also see more complicated in newlib (https://github.com/riscv/riscv-newlib/blob/riscv-newlib-2.5.0/libgloss/riscv/crt0.S) and glibc (https://github.com/riscv/riscv-glibc/blob/riscv-glibc-2.26/sysdeps/riscv/start.S), though those both assume that the stack has already been initialized by the execution environment.

Upvotes: 5

Pavel Smirnov
Pavel Smirnov

Reputation: 219

There is SCR1 - an open-source RISC-V compatible core (https://github.com/syntacore/scr1) with SDK (https://github.com/syntacore/scr1-sdk, please, clone with submodules, recursively).

You can find sample applications in the SDK.

The startup code is located in the bootloader (https://github.com/syntacore/sc-bl).

See label _crt_start in src/startup.S:

  • init CSRs
  • clear .bss
  • init stack
  • init TLS
  • boot
  • call main

Upvotes: 2

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